1. Field of the Invention
This invention pertains to blowout preventer instrumentation and more particularly to the direct measurement of position, velocity, and rate of movement of the parts in a ram type blowout preventer.
2. Description of the Prior Art
Blowout preventers (BOP's) are employed at the wellhead of an oil or gas well as a safety device for shutting off the flow of fluids from the annulus of the well in the event of excessive build-up of pressure. Although there are many kinds of blowout preventers, one popular class is known as a "ram" type blowout preventer. In this type of blowout preventer, the well pipe extends through a central opening in the preventer housing or block. Guideways, usually two opposing ones, in the housing of the preventer are located transverse to the axis of the well pipe. Rams driven by hydraulically actuated pistons operate with respect to these guideways to reciprocate the rams to close off the annulus around the pipe when there is a need. Shear ram blowout preventers include rams having sharp edges for shearing off the pipe, rather than closing around it, so that these rams completely shut off the annulus and the pipe opening, as well, when they are closed.
Knowledge of the parameters of the conditions of the well at the blowout preventer is extremely important to maintaining proper operation of the well. From these parameters, the well can be controlled so that safe conditions can be maintained in ordinary circumstances of operation and when an unsafe condition is detected that cannot be controlled, shut down of the well can be appropriately initiated, either manually or automatically. For example, pressure transducers and temperature transducers are used at the blowout preventer to develop signals indicative of these respective conditions. These and other signals are typically presented as control signals on a control console employed by a well operator. This operator can, for example, control the operations of the well by con-trolling the rotating speed on the drill pipe, controlling the downward pressure on the drill bit, regulating the circulation pumps for the drilling fluid for a drilling operation and/or adjusting the extent that the annulus is opened or closed by actuating the hydraulically actuated pistons connected to the reciprocating rams.
Heretofore, it has not been possible to know precisely the location of the rams, even though the operator might be controlling them. The control operator has had to rely on secondary measurements, such as well flow, to estimate the positions of the rams. Well flow, however, is affected by much more than how large the ram gap is. Thus, there has been a need for an improved measure of the ram gap in a more direct manner.
One device that has been employed in the past to develop a signal indicative of the relative position of component parts located in an enclosed housing (not necessarily in a blowout preventer housing) is a potentiometric transducer. Such a device employs one or more sensors that are subject to wear and inaccuracies in the presence of a harsh environment. Moreover, such sensors are subjected to being lifted from the surface of whatever is being tracked, which causes inaccuracies. Also, a loss of power often causes distorted readings because these devices operate incrementally, adding or subtracting values related to specific turns or segments of wire to a previous value. Moreover, devices such as these are notoriously poor high speed devices. Thus, potentiometric measurement would not be useful in accurately determining the position parameter of ram movement. In addition, since the velocity and rate conditions of ram movement are also important, potentiometric devices are not useful since they cannot provide, however imperfectly, such information at all.
It is further noted that incremental measuring devices of whatever sort for only keeping track of steps have the inherent shortcoming of having to be reset to a baseline in the event of a power failure as well as not providing the precision that is attendant to continuous measurement.
Therefore, it is a feature of the present invention to provide an improved apparatus for precisely measuring the location or position of a ram or ram piston in a blowout preventer.
It is another feature of the present invention to provide an improved instrumentation device that continuously and absolutely determines the position of a ram or a ram piston in a blowout preventer and which does not have to be reset in the event of power failure.